The invention relates to the analysis of the fluid in an oil well and more particularly to the measurement of the photon absorption properties of the well fluids such measurement providing indications as to the composition of the fluid.
There is disclosed in U.S. Pat. No. 2,961,539 to Egan et al a sonde for determining the density of fluid in a well bore, comprising a source of gamma rays and a gamma ray detector spaced from the source, with a gamma ray absorbing shield disposed in the direct path from the source to the detector. The flux produced by the source thus irradiates the fluid outside the sonde.
It is pointed out in this patent that some of the photons striking the detector have been scattered from the formation traversed by the well back to the detector. Under such conditions, an accurate measurement cannot be obtained.
In order to deal with this problem, the above patent teaches disposing about the source and the detector a tubular shield, e.g. of lead, to absorb the photons back-scattered from the formations.
However, the presence of such a shield is disadvantageous in that it creates a fluid path separate from the main flow, only this fluid channel being subject to measurement. In this connection, it should be pointed out that the composition of the fluid inside this channel may depart from the compositon in the main flow. This occurs particularly in deviated wells because differences in density of the components of the fluid bring about segregation of these components over the cross-section of the well. This, again, prevents accurate measurements from being obtained.
Another proposal, described in the above patent with reference to FIG. 7 (see also U.S. Pat. Nos. 3,103,812, 3,123,709, 3,688,115; French Pat. Nos. 1,283,068, 1,291,856), consists of measuring the absorption of gamma rays over the direct path from the source to the detector. This requires that a fluid channel be created inside the sonde, with gamma ray absorbing material encompassing the fluid channel. This solution suffers from the same defect as that mentioned above, i.e. the measurement yields information about a very limited sample of the well fluid, which may not be representative of the main fluid flow insofar as the composition is concerned.
In addition, all of the above-mentioned patents merely disclose that the purpose of gamma ray absorption measurements is to determine the density of the well fluid. The fluid in an oil well generally contains a mixture of oil and water, generally saline water. Oil and water have densities which are rather similar, for instance 1 or slightly more for water and 0.8 for oil. This low contrast represents a major limitation in the accuracy of the data obtained. Therefore, when only density data are derived from the measurements of gamma ray absorption and the water/oil ratio is computed from those data, the precision of the result will be mediocre. Moreover, no valuable indication as to salinity can be drawn from density data since the variation of density with salinity is very small.
In Soviet Union Author's Certificate No. 326,904, a method for the determination of salinity is disclosed, in which, in addition to the conventional absorption measurement, the X-fluorescence line of chlorine is detected to determine the amount of chlorine in the water and therefore the salinity. This method is impractical because the fluorescence line of chlorine lies at 2.8 kev, well below the minimum energy capable of being detected by the photon detectors usable in well logging sondes, which is about 20 kev.